Spring-loaded solenoid-based actuators are often employed to control locks or the flow of fluids, for example. However, they are typically monostable devices and require a continuous current to maintain the driving rod of the device in its actuated position. This leads to unwanted energy dissipation in the form of heat.
EP-A-1119723 (filed by the present applicant under reference 554.02/W) describes a magnetic drive having a bistable characteristic, which can be configured to revert to (or remain in) one of its two states in the event of a power failure.
U.S. Pat. No. 3,772,540 relates to an electromechanical latching actuator for producing linear or rotary motion. FIGS. 1A to 1D depict an actuator which includes one or more sets of radially polarised permanent magnets and electric coils which annul and flux switch a magnetic field between adjacent magnetically isolated poles, thereby sequentially generating a force or torque that can be coupled to a suitable load. However, its performance may be affected by magnetic fields present in its surrounding environment.
The present invention seeks to provide a robust and reliable electromagnetic actuator configuration, suitable for use in a broad range of applications.
The present invention provides an electromagnetic actuator comprising an armature comprising a permanent magnet, wherein the armature is movable between first and second stable positions; two electric coils disposed on opposing sides of the armature along its direction of movement, with their axes substantially aligned with said direction; and a magnetic flux container substantially surrounding the armature and the coils to substantially contain magnetic flux generated thereby and to substantially shield its interior from external magnetic flux, wherein in each stable position magnetic flux generated by the permanent magnet extends around a magnetic circuit path including the container so as to retain the armature in its stable position, and wherein energising the coils causes the armature to move from one stable position to the other.
The known actuator configurations acknowledged above have open flux arrangements wherein the permanent magnets create flux which extends outside the actuators themselves. Therefore their performance is susceptible to external influences. For example, it may be influenced by magnetic surrounding components such as another actuator or a ferromagnetic housing. In addition, an open magnetic field attracts ferromagnetic particles from the environment. A fluid or gas flowing close to the actuator may include small ferromagnetic particles, for example as the result of corrosion. Aggregation of such particles risks causing a blockage. This is undesirable in many applications, particularly critical roles in jet engine fuel flow control or the space industry for example.
The magnetic flux container present in an actuator according to the invention extends around the armature and electric coils in such a way as to substantially contain within it the magnetic flux generated by these elements, thereby minimising any side effects resulting from flux leakage. Magnetic circuits formed during operation of the device are closed by the container.
Furthermore, the container serves to shield the interior of the actuator from external magnetic fields. The actuator is substantially sealed against the ingress of magnetic flux from outside by the container.
Preferably, each coil is wound round a coil core which forms part of the magnetic circuit created when the armature is adjacent to the respective coil. More particularly, the actuator may be configured such that, when the armature is in either of the stable positions, the shortest path from the armature to the container is less than the shortest path from the armature to the more distant of the two coil cores. This ensures that the armature is reliably latched against one of the coil cores in each stable rest position.
The armature may include pole pieces on opposing sides of the permanent magnet along its direction of movement. The actuator is preferably configured such that, when the coils are energised, the path of magnetic flux through the pole piece closest to the corresponding coil core changes from a substantially axial orientation to a substantially radial orientation, and vice versa for the other pole piece.
In preferred embodiments, each pole piece defines a surface for engagement with a respective coil core, and each coil core defines a complementary engagement surface.
In particular, each of said pole piece engagement surfaces may include a frustoconical portion. This serves to create a more uniform force of attraction characteristic between the two mating surfaces, relative to planar faces.
In a preferred implementation, the permanent magnet is orientated with its North and South poles aligned with the direction of movement of the armature. Relative to radial alignment of the poles, a significantly greater locking force is achieved as a greater area of high flux density faces the adjacent coil core.
Actuators embodying the invention preferably include an energy storage arrangement for storing energy derived from movement of the armature into each stable position. This storage arrangement transfers energy to the armature as it moves away from each stable position. This provides internal energy recycling and so reduces the power required to switch the device. It also affords a “soft landing” effect, which will extend the lifetime of the actuator. Also, in applications where the actuator controls fluid flow by pinching a deformable tube, the deceleration caused by the energy storage arrangement as the actuator moves towards each stable position reduces the likelihood of damage to the tube.
The extent of the energy storage may be readily adjusted as appropriate to alter the net latching force exerted on the armature to suit different applications.
The energy storage arrangement may comprise a pair of resilient devices, such as coil springs for example, with one of the devices being compressed or extended as the armature moves into a respective stable position. The resilience of these devices may be selected to suit a particular requirement.
Each resilient device may be disposed between a pole piece and a respective coil core, providing a compact and self-contained configuration. Alternatively, the resilient devices may be located outside the housing of the actuator to provide a greater area of engagement between the armature and the coil cores, thereby increasing the latching force. Also, larger resilient devices may be more readily accommodated outside the actuator housing in this implementation.
In some embodiments, either resilient device is only compressed or extended as the armature moves through a final portion of its travel into a respective stable position.
In a further embodiment of the invention, the actuator has a third stable position between the first and second stable positions. This third position is preferably defined by spring and passive magnetic forces acting on the armature.
A pair of resilient devices may be arranged such that one of them is compressed (or extended) or compressed (or extended) further if the armature moves away from the third stable position, so as to urge the armature towards the third stable position.
Preferably, each resilient device is partially compressed (or extended) when the armature is in the third stable position. This pre-loading of each resilient device makes the third stable position more definite and more clearly defined and readily selectable.
The extent to which each resilient device may be partially compressed (or extended) when the armature is in the third stable position may be adjustable so as to emphasise the third position to the degree needed to meet particular requirements.
According to a further preferred configuration, an actuator may be arranged such that when the armature moves from the third stable position to one of the first and second stable positions so as to compress (or extend) further one of the resilient devices, at least during a final portion of said movement (preferably substantially the whole of said movement), the degree of partial compression (or extension) of the other resilient device remains substantially unchanged. This has the effect that during movement of the armature from the third stable position to another stable position and back again, energy is not expended in deformation of the other resilient device and it does not therefore influence this action of the actuator.
Conveniently, the magnetic flux container may form the housing of the actuator.
According to a further aspect, the present invention provides a method of operating an actuator as described herein, comprising the step of moving the armature from one stable position to the other by energising the coils so as to generate axial magnetic flux through each coil in respective opposite directions. As will be described with reference to embodiments of the invention below, applying a current pulse momentarily to each coil in this manner serves to substantially nullify the flux created by the permanent magnet on one side whilst augmenting the flux density on the other side, causing the armature to switch positions.
The armature is held in each stable rest position by spring and/or passive magnetic forces alone, with only a brief current pulse needed as and when the actuator is switched to a different stable rest position. Its power consumption is therefore very low.